1. Field of the Invention
This invention relates generally to electronic printed circuit boards, and more specifically, to a printed circuit board having an embedded high capacitance core used as a decoupling capacitor for integrated circuits installed on the printed circuit board.
2. Related Art
Typical printed circuit boards consist of multiple layers of composites of organic and inorganic materials, with internal and external wiring, allowing electrical components to be mechanically supported and electrically connected. The technology trend in printed circuit board manufacturing has been towards increasing numbers of interconnect layers, greater pass-through hole densities (permitted by smaller hole diameters), and finer trace widths. The goal of this trend is to allow greater numbers of components to be installed on a printed circuit board of a given size. Throughout this evolution in printed circuit board technology, however, little progress has been made with respect to building active or passive electronic devices as integrated elements during the multi-layer printed circuit board manufacturing process. This lack of progress is due primarily to the incompatibility of printed circuit board manufacturing processes and component manufacturing processes. Few process similarities exist.
In a typical multi-layered printed circuit (PC) board, a plurality of layers or planar, copper foil-clad fiberglass elements are laminated together, with certain of the copper surfaces having been previously etched to provide the desired signal paths between externally mounted components. Power is typically supplied to the externally mounted components, such as integrated circuits, via a power distribution core consisting of a fiberglass/epoxy dielectric element sandwiched between a copper foil power plane and a similar ground plane.
The dielectric constant of the fiberglass/epoxy dielectric typically ranges from approximately 4.0 to 5.5. Such a low dielectric constant provides a capacitance of approximately 200 picoFarads per square inch. This capacitance is not sufficient to satisfy the inrush (instantaneous) current requirements of a typical integrated circuit. Therefore, a fiberglass/epoxy power core necessitates that additional, externally mourned decoupling capacitors be used on the printed circuit board.
For a typical printed circuit board assembly having a large number of integrated circuits, a correspondingly large number of decoupling capacitors is typically required. Such capacitors occupy considerable printed circuit board surface area, require extra assembly (insertion) time, and increase the overall cost of a completed primed circuit board unit. In addition to the per capacitor cost, each capacitor also constitutes a potential point of failure that reduces system reliability.
Commonly owned U.S. Pat. No. 5,162,977 to Paurus et al. discloses a printed circuit board which includes a high capacitance power distribution core. The high capacitance core consists of a ground plane and a power plane separated by a dielectric element having a high dielectric constant. The dielectric element is typically glass fiber impregnated with a binding/bonding material, such as an epoxy resin loaded with a high dielectric constant, fired ceramic substance. The ceramic substance is in the form of a nanopowder. The resulting capacitance of the power distribution core is typically sufficient to eliminate, or significantly reduce, the need for decoupling capacitors.
Conventional glass fibers used in power cores have a dielectric constant of approximately 6.0. Conventional epoxy resins have a dielectric constant of approximately 4.0. The dielectric constant of the resulting combined glass fiber/epoxy resin core is in the range of 4.2 to 4.6. In the '977 patent disclosure, two materials having widely differing dielectric properties are combined (i.e., a high dielectric constant binder and a relatively low dielectric constant fiberglass mat). The resultant effective dielectric constant of the combination varies considerably with the ratio of the two materials.
Commonly owned, copending U.S. patent application No. 08/175,048, filed Dec. 29, 1993, is directed to improving upon the teachings of the '977 patent by providing a dielectric element in which both the fiber mat and the binder have a high dielectric constant. Thus, a high dielectric composite material is formed. The present invention is directed toward improving further on the teachings of the '977 patent to provide a high capacitance power distribution core which is inexpensive and straightforward to manufacture.